Zero-benzene aromatics complexes are attractive to purified terephthalic acid producers because more of the feedstock is converted to the desired product (para-xylene) as opposed to a side product (benzene). Toluene methylation technology enables zero-benzene aromatics complexes. The current flow scheme converts all the toluene in a toluene methylation unit, and all the benzene and A9/A10 aromatics in a trans-alkylation unit. In the current flow scheme, the trans-alkylation unit converts benzene and A9/A10 aromatics to a trans-alkylation product comprising benzene through A11+ aromatics with toluene and xylenes as the focus. The problem with the current flow scheme is that some feed cases require very large recycle loops through fractionation columns in order to convert the benzene and A9/A10 aromatics to extinction. These recycle loops require high costs. This is particularly the case when there is extra benzene in the feed (e.g., an aromatics complex with an external benzene feed or integrated with benzene producing technologies such as reforming or dehydrocyclodimerization) or when there is extra A9/A10 material in the feed or produced in one of the aromatics complex units (e.g., toluene methylation).
In one embodiment, the disclosure addresses a case where extra benzene and toluene are produced or brought into the existing aromatics complex. The stoichiometry of benzene and toluene relative to A9+ in the feed is significantly below methyl to phenyl ratio of 1.5 in the current flow scheme. This leads to low per-pass conversion of benzene in the trans-alkylation unit, resulting in very high benzene recycle. This situation would call for more efficient and balanced methylation vs. trans-alkylation unit operations to minimize the benzene and toluene recycle. In this disclosure, at least a portion of the benzene is diverted from the trans-alkylation unit and sent to the toluene methylation unit. Benzene reactivity is shown to be at least half of the toluene reactivity in a toluene methylation unit.
In another embodiment, the disclosure addresses a case where extra A9+ materials are produced or brought into the existing aromatics complex. The stoichiometry of benzene and toluene relative to A9+ is significantly above methyl to phenyl ratio of 1.5 in the current flow scheme. This leads to low per-pass conversion of A9+ in the trans-alkylation unit, resulting in very high A9+ recycle. This situation would call for more efficient and balanced methylation vs. trans-alkylation unit operations to minimize the A9+ recycle. In this disclosure, at least a portion of the toluene is diverted from the toluene methylation unit and sent to the trans-alkylation unit. The trans-alkylation unit converts benzene, toluene, and A9/A10 aromatics at high per-pass conversion to a trans-alkylation product comprising benzene through A11+ aromatics with toluene and xylenes as the focus.